The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, catamenial products such as sanitary napkins, and disposable medical products such as bandages and dressings, is the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. Thinner articles are also more compact in the package, making the articles easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per absorbent article.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain particulate absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" materials has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such particulate absorbent polymers in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these particulate absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and particulate absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These particulate absorbent polymers have the ability to retain large volumes of aqueous body fluids, such as urine. A representative example of such particulate absorbent polymers are lightly crosslinked polyacrylates. Like many of the other absorbent polymers, these lightly crosslinked polyacrylates comprise a multiplicity of anionic (charged) carboxy groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb aqueous body fluids as the result of osmotic forces.
Besides osmotic forces, absorbency based on capillary forces is also important in many absorbent articles, including diapers. Capillary forces are notable in various everyday phenomena, as exemplified by a paper towel soaking up spilled liquids. Capillary absorbents can offer superior performance in terms of the rate of fluid acquisition and wicking, i.e. the ability to move aqueous fluid away from the point of initial contact. Indeed, the dual-layer core absorbent structures noted above use the fibrous matrix as the primary capillary transport vehicle to move the initially acquired aqueous body fluid throughout the absorbent core so that it can be absorbed and retained by the particulate absorbent polymer positioned in layers or zones of the core.
Other absorbent materials which provide capillary fluid transport capabilities are open-celled polymeric foams. If made appropriately, open-celled polymeric foams provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores for absorbent articles such as diapers. Absorbent articles containing such foams possess desirable wet integrity, provide suitable fit throughout the entire period the article is worn, and avoid degradation in shape during use. In addition, absorbent articles containing such foam structures are easier to manufacture on a commercial scale. For example, absorbent diaper cores could simply be cut out of continuous foam sheets and could be designed to have considerably greater integrity and uniformity than air-laid fibrous absorbent cores containing particulate absorbent polymers. Such foams could also be molded in any desired shape, or even formed into integral, unitary diapers.
Literature and commercial practice is replete with descriptions of various types of polymeric foams that can imbibe a variety of fluids for a variety of purposes. Indeed, employment of certain types of polymeric foam materials as elements of absorbent articles such as diapers and catamenial products has previously been suggested. See, for example, U.S. Pat. No. 4,029,100 (Karami), issued Jun. 14, 1977, that discloses a shape-retaining diaper that can employ a foam element in the crotch area of its absorbent pad assembly in order to provide high wet resiliency. Certain types of polymeric foam materials have also been suggested as useful in absorbent articles for the purpose of actually imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,528 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structures such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams). U.S. Pat. No. 5,147,345 (Young et al.), issued Sep. 15, 1992, illustrates possible absorbent article configurations which are suitable for use with absorbent members formed of such polymeric foam materials, and is hereby incorporated herein by reference.
Although various polymeric foam materials have been suggested for use in absorbent articles, absorbent, open-celled polymeric foams have been developed which have the following desirable characteristics:
(a) a relatively greater affinity for absorbing body fluids than exhibited by other components in the absorbent article so that the foam material can drain (partition) fluids from these other components and keep the fluids stored within the foam structure; PA1 (b) relatively good wicking and fluid distribution characteristics in order for the foam to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure, thus allowing for subsequent gushes of fluid to be accommodated; and PA1 (c) a relatively high storage capacity with a relatively high fluid capacity under load, i.e. under compressive forces.
As previously noted, a thinner absorbent core is usually a requirement for making relatively thin absorbent articles, such as diapers. However, providing absorbent polymeric foam materials that remain relatively thin in form until wetted with aqueous body fluids is not straightforward. The absorbent foam material needs to remain relatively thin during normal storage and use prior to being wetted. This relatively thin polymeric foam material must additionally have the needed absorbency characteristics described above if it is to be useful in high performance absorbent cores. Making relatively thin polymeric foams that are sufficiently soft and flexible for comfort of the wearer is also not a trivial task.
Relatively thin, collapsed (i.e. unexpanded), polymeric foam materials have been developed that, upon contact with aqueous body fluids, expand and absorb such fluids. These absorbent polymeric foam materials comprise a hydrophilic, flexible, nonionic polymeric foam structure of interconnected open-cells. Such collapsed polymeric foam materials remain relatively thin during normal shipping, storage and use conditions, until ultimately wetted with aqueous body fluids, at which point they expand. Because of their excellent absorbency characteristics, including capillary fluid transport capability, these collapsed polymeric foam materials are extremely useful in high performance absorbent cores for absorbent articles such as disposable diapers, adult incontinence pads or briefs, catamenial products such as sanitary napkins, disposable medical products such as bandages and dressings, and the like. Such collapsed polymeric foam materials are described in commonly assigned, co-pending U.S. patent application Ser. No. 07/989,270 (Dyer et al., filed Dec. 11, 1992), the disclosure of which is hereby incorporated herein by reference.
Although such polymeric foam materials exhibit superior storage capacity, and the advantageous thin-until-wet feature, they are often slower to acquire fluid than other materials. As a result, sudden gushes of fluid may simply run off of the surface of such an absorbent member with comparatively little fluid actually being absorbed and stored. This in turn may lead to premature product leakage before the absorbent capacity of the article is fully utilized. In addition, the use of comparatively thicker slices or additional layers of polymeric foam material to increase storage capacity increases the resulting stiffness of the absorbent article, without improving the acquisition rate.
Accordingly, it would be desirable to be able to make an absorbent member that: (1) has adequate or preferably superior acquisition characteristics, so as to be desirable in high performance absorbent cores used in absorbent articles such as disposable diapers, adult incontinence pads or briefs, catamenial products such as sanitary napkins, disposable medical products such as bandages and dressings, and the like; (2) is sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article; (3) utilizes a combination of mechanical features and material properties to produce advantageous dynamic structures; and (4) can be manufactured on a commercial scale, at relatively reasonable or low cost.